In the industrial field the operation of cutting is a mechanical operation by means of which one piece of metal is detached from another by working with special tools which are capable of cutting.
This operation is performed by applying a suitable pressure with the tool (blade or punch) in one area of the material so as to make it slide with respect to the others.
The tool used is a blade if the cuts to be made are open but it is a punch if-the cuts are of closed profile.
The cutting is a very rapid operation. It produces well-defined shapes in an instant and is particularly suitable for the working of metal sheets.
Opposite to the tool, which has a rectilinear movement, is the die, in fixed position, which functions as support for the metal sheet and enables it to be cut.
Initially the tool displays its action with a compression of the material, causing a plastic deformation to take place which creates a hollow body; in the subsequent phase, the compression force, which increases gradually, reaches and exceeds the value of the shear strength thus producing the detachment of a portion of metal sheet.
During the cutting operation, the fibers of the metal sheet, adjacent to the cutting corner of the punch, are bent following the movement of the tool for a short path prior to the stage when they are finally cut, so that they are compressed and deformed along the entire cut contour.
In particular, because of the elasticity of the material, internal reactions occur, in correspondence with the cut fibers, and these reactions cause the appearance of appreciable friction between the slide walls and this friction prevents the exit of the cut disc from the die and the removal of the punch from the hole of the metal sheet.
For the purpose of avoiding these drawbacks, a blank-holder is used which, by acting by compression in the vicinity of the edge to be cut, from the time prior to the action of the punch and until the latter is completely retracted, prevents the deformation of the cut fibers.
In the current state of technology, the industrial machines which perform the cutting of metal sheets by means of punching are so equipped as to have a punching assembly essentially constituted by the punch itself, the die underneath and a flexible element which moves the above mentioned punch.
The punch is held in a guide and can slide in the interior of a body known as a punch guide which also has the function of holding, at its lower end, the eventual blank-holder element should the use of this be required.
In its turn, the punch guide can slide within the seat machined on the bearing structure of the machine.
Structurally speaking the punching assembly therefore has two coaxial cylindrical profiles, in each of which is provided the coupling between the surface of the body which constitutes the hole and the surface of the body which constitutes the shaft. This coupling, being of the guide type, therefore requires that the two surfaces have to be machined to tolerance and with a very low degree of roughness.
Furthermore, in correspondence with each of the above mentioned profiles, a mechanical element, a key or other element, must be provided which is capable of preventing the relative displacement between the element which constitutes the hole and the element which constitutes the coupling shaft.
From the above it will therefore be apparent that the production of a punch assembly requires the machining, to tolerance and with a minimum degree of roughness, of at least four surfaces, i.e. those constituting the two couplings and the creation of at least two seats for the housing of the associated stop keys; all this involving increased costs requiring considerable accuracy between the die and punch.